The production of mineral oils, lube oils, transformer oils, other specialty oils and various and sundry fuels and other products involves, in addition to distillation and extraction, the process of dewaxing. This process is necessary to remove the paraffinic (wax) components of the oil or fuel involved, thus facilitating its use for various purposes under a variety of conditions and environments including low temperatures. In general, it has been found that there are two wax components in each feed stock that must be removed to attain the required oil or fuel properties, the high melting point waxes (or hard waxes) and the low melting point waxes (or soft waxes).
Dewaxing of oils and fuels can be done in two different ways. The first approach involves the physical, nondestructive separation of the wax (or paraffin) molecules from the oil by crystallization.
Simple dewaxing can be practiced merely by reducing the temperature of the stock to be dewaxed to a low enough point so as to induce the crystallization of the wax. This is practical when the wax to be removed is relatively high melting, when the amount to be removed is relatively small, and/or when the oil being dewaxed is relatively light, i.e., does not become unmanageable due to high viscosities at low temperature.
Alternatively and preferably, the stocks are dewaxed by solvent dewaxing processes, which are well known in the art, in which solvent and feed are chilled to crystallize the wax which is then removed. These processes employ various solvents which reduce the solubility of the wax in the oil and also exert a dilution effect on the feed stock.
The simplest solvent dewaxing process involves the introduction of an appropriate solvent, at an appropriate volume (dilution) into the stock to be dewaxed, forming a mixture, and reducing the temperature of the mixture to the desired wax filtration temperature. This procedure is exemplified by typical incremental dilution processes using scraped surface chillers. The use of scraped surface chillers, however, is plagued with the problems of deposition of precipitated wax on the heat exchanger surfaces with the resulting pressure drop and the crushing of the wax crystals by the scraper blades which contribute to poor filtration.
As a response to these problems the incremental dilution chilling process was developed. The DILCHILL (DILCHILL is a registered service mark of Exxon Research and Engineering Company) dewaxing process involves the cooling of the waxy hydrocarbon oil stock in an elongated stirred vessel, preferably a vertical tower, with a prechilled solvent that will solubilize at least a portion of the oil stock while promoting the precipitation of the wax. Waxy oil is introduced into the elongated staged cooling zone or tower at a temperature above its cloud point. Cold dewaxing solvent is incrementally introduced into said zone along a plurality of points or stages while maintaining a high degree of agitation therein so as to effect substantially instantaneous mixing of the solvent and wax/oil mixture as they progress through said zone, thereby precipitating at least a portion of the wax in said oil. DILCHILL dewaxing is discussed in greater detail in the U.S. Pat. Nos. 3,773,650 and 3,775,288.
To complete the solvent dewaxing processes two main separations are involved, namely the separation of the precipitated wax crystals from the solvent-oil mixture and then the separation of the solvent from the oil.
The prior art extensively describes the use of filters, in particular rotary filters, for wax crystal removal from solvent-oil mixtures and presents a large number of modifications to improve separation efficiency. Despite these improvements, filters still suffer from the fact that the wax cake they produce contains a large amount of liquid. This liquid (solvent and oil) means loss of yield (oil lost with wax) and requires the expenditure of much energy in distillation so as to recover solvent from the wax for recycle. These facts and drawbacks have led to the use of two or more filters in series to increase yield. The use of centrifuges for this wax/solvent-oil separation has also been considered. Filtering centrifuges which are a direct extension of filters have been used. They are, however, limited in usefulness to recovering dewaxing solvents denser than the wax (e.g., chlorinated solvents). Centrifuges are useful in wax crystal separation provided that trace wax contamination in the centrate (liquid overlow from the centrifuge) can be tolerated.
The separation of oil from solvent is commonly done by standard distillation techniques. Over the years this separation has been improved by new tower design and heat integrations. Despite these improvements, however, distillation of solvent from oil is still very energy intensive. Among the reasons for this is the fact that very large volumes of solvent must be distilled (4 to 6 volumes solvent/volume oil) and that no matter how well the heat integration is performed, efficiency is seldom above 60%.
The use of membrane processes for liquid/liquid separations have received attention recently (see A. S. Michaels 7th World Pet. Conf. 4 21, 1967), the main reason being their energy efficiency, which comes about because they do not require phase changes (e.g., liquid to vapor as does distillation) to effect the separation. While most of the effort on the use of membranes has been limited to aqueous systems, some studies of the liquid permeation and separation of non-aqueous mixtures have been reported. European Patent Application EP No. 13,834 describes a membrane and a process using said membrane, whereby solvent can be partially (50%) removed from oils. Other patents have described the use of membranes for hydrocarbon separations. See for example U.S. Pat. No. 4,154,770, U.S. Pat. No. 3,043,891 and U.S. Pat. No. 2,970,106.
The second approach to waxy feedstock dewaxing is a relatively new technique that relies on destructively removing the wax molecules. This is done by subjecting the wax molecules to a catalyst in the presence of hydrogen, whereupon the wax molecules in the feed are cracked to smaller molecules, or isomerized.
U.S. Pat. No. 3,700,585, now U.S. Pat. No. Re. 28,398, which deals with the dewaxing of oils by shape selective cracking and hydrocracking over specific zeolite materials, is fairly representative of this type of process. Other patents which deal with catalytic dewaxing are U.S. Pat. Nos. 3,980,550; 3,893,906; 3,968,024; 4,137,148; 3,663,430; 4,176,050; 4,181,598. U.S. Pat. No. 3,755,138 and Canadian Pat. No. 903,696 disclose the combinations of solvent and catalytic dewaxing.
Catalytic dewaxing suffers from the disadvantage that wax molecules are not recovered, but rather destroyed. Catalytic dewaxing must be conducted under conditions severe enough to crack the hard, high melting waxes. Its main advantage, however, is its relatively lower cost in both initial investment and operation.
In summary then, we see that there are basically two dewaxing schemes: solvent dewaxing which requires large volumes of solvent, low temperature and is energy intensive, but produces a wax product which can be sold; and catalytic dewaxing which is relatively simple, inexpensive, but requires hydrogen, must be severe enough to remove hard wax, and removes wax by destructive conversion. We also see that in solvent dewaxing following the wax crystallization, additional steps are required to effect liquid/solid and liquid/liquid separations and these involve (a) filters which leave substantial volumes of solvent in the wax; (b) centrifuges which reduce the quantities of liquid in the wax (thus being an improvement in yield and energy use), but are mostly suitable for solvents denser than wax and suffer from the possibility that trace wax may be present in the liquids (centrate overflow); (c) distillation which is energy intensive,; or (d) membranes which are not energy intensive.
U.S. Pat. No. 3,078,222 describes a process involving catalytic hydrogenation of aromatics and ring scission followed by solvent dewaxing. Catalytic dewaxing followed by centrifugation is described in U.S. Pat. No. 3,654,128. U.S. Pat. No. 3,755,138 teaches solvent dewaxing followed by catalytic dewaxing. None of these patents, however, suggest the combined solvent dewaxing-catalytic dewaxing process in combination with centrifuge membrane solvent recovery which is the present invention.